Light-emitting devices with textured active layer

ABSTRACT

A device includes a textured substrate having a trench extending from a top surface of the textured substrate into the textured substrate, wherein the trench comprises a sidewall and a bottom. A light-emitting device (LED) includes an active layer over the textured substrate. The active layer has a first portion parallel to the sidewall of the trench and a second portion parallel to the bottom of the trench.

PRIORITY DATA

This application is a Divisional application of Ser. No. 12/720,462,filed on Mar. 9, 2010, entitled “LIGHT-EMITTING DEVICES WITH TEXTUREDACTIVE LAYER,” the disclosure of which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

This disclosure relates generally to light-emitting devices (LEDs), andmore particularly to the methods of manufacturing LEDs with texturedactive layers and the resulting structures.

BACKGROUND

Light-emitting devices (LEDs), such as light-emitting diodes or laserdiodes, are widely used for many applications. As is well known to thoseskilled in the art, an LED may include a semiconductor light-emittingelement having a plurality of semiconductor layers formed on asubstrate. The substrate may be formed of, for example, galliumarsenide, gallium phosphide, alloys thereof, silicon carbide, and/orsapphire. Continued development in LEDs has resulted in highly efficientand mechanically robust light sources that can cover the visiblespectrum and beyond. These attributes, coupled with the potentially longservice life of solid state devices, may enable a variety of new displayapplications, and may place LEDs in a position to compete with the wellentrenched incandescent and fluorescent lamps.

LEDs emit light from active layers. Accordingly, light may be extractedfrom the opposite sides of the active layers. However, in certainsituations, for example, when an LED is used for illumination, it may bepreferred that the light is directed to only one side of the LED, withthe light being scattered to achieve a more uniform light distribution.Conventionally, patterned package substrates were bonded to LED chips tore-direct light to desirable directions. This solution, however, resultsin an increase in the cost and complexity in the formation of thepackage substrates and the bonding process for bonding LED chips to thepackage substrates. In addition, the solutions in package substrates didnot help improve the light-extraction efficiency.

SUMMARY

In accordance with one aspect, a device includes a textured substratehaving a trench extending from a top surface of the textured substrateinto the textured substrate, wherein the trench comprises a sidewall anda bottom. A light-emitting device (LED) includes an active layer overthe textured substrate. The active layer has a first portion parallel tothe sidewall of the trench and a second portion parallel to the bottomof the trench.

Other embodiments are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 5 are cross-sectional views and top views ofintermediate stages in the manufacturing of a light-emitting device(LED) in accordance with an embodiment, which LED is formed on atextured substrate.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative and do not limit the scope of the disclosure.

A novel light-emitting device (LED) in accordance with an embodiment andthe method of forming the same are presented. The intermediate stages ofmanufacturing an embodiment are illustrated. The variations and theoperation of the embodiment are then discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

Referring to FIG. 1, chip 100, which comprises substrate 20, isprovided. Chip 100 may be a part of an un-diced wafer comprising aplurality of identical chips. In an embodiment, substrate 20 is formedof sapphire (Al₂O₃). In alternative embodiments, substrate 20 is asilicon-containing substrate, such as a silicon carbide substrate orsilicon. In yet other embodiments, substrate 20 comprises compoundsemiconductor materials comprising group-III and/or group-V elements, oralso known as III-V compound semiconductor materials.

Photo resist 22 is formed on substrate 20 and is patterned to form aplurality of openings 24, as shown in FIG. 2. In an embodiment, thehorizontal dimensions (widths or lengths, for example, W1, W2, and W3)of openings 24 may be different from opening to opening, although thehorizontal dimensions of openings 24 may also be the same throughoutchip 100. Further, spacings (for example, S1 and S2) between neighboringopenings 24 may be the same or different from each other. Accordingly,the pattern densities of openings 24 may have variations in differentregions of chip 100, wherein the pattern density of openings 24 in aregion equals the total area of openings 24 in the region divided by thetotal area of the region. In the embodiment wherein the horizontaldimensions and spacings of openings 24 are substantially uniformthroughout chip 100, the pattern densities of openings 24 aresubstantially uniform throughout chip 100.

Referring to FIG. 3A, substrate 20 is etched through openings 24 to forma plurality of discrete trenches 26, which extend from top surface 20 aof substrate 20 into substrate 20. The resulting substrate 20 isreferred to as a textured substrate hereinafter. The resulting surfaceof substrate 20 that includes trenches 26 is referred to a textured topsurface, which comprises portions of the original top surface 20 a,sidewalls 26_1 of trenches 26, and bottoms 26_2 of trenches 26. In anembodiment, sidewalls 26_1 and bottoms 26_2 are substantially straightin the cross-sectional view, although they may also be curved. Theetching may be a dry etching, although other etching methods may also beused. Through the adjustment of process conditions of the etchingprocess, slant angles α (illustrated as α1, α2, α3, and so on) ofsidewalls 26_1 may be adjusted to less than about 45 degrees and greaterthan about 1 degree (or greater than about 10 degrees or 15 degrees)from horizontal, or between about 10 degrees and about 45 degrees fromhorizontal, or even between about 15 degrees and about 35 degrees fromhorizontal. Photo resist 22 is then removed.

FIG. 3B illustrates a top view of the structure shown in FIG. 3A,wherein the cross-sectional view as shown in FIG. 3A is obtained in theplane crossing line 3A-3A in FIG. 3B. In an embodiment, trenches 26 haverectangular shapes with the respective widths and lengths close to eachother (and hence trenches 26 have the shape of squares) in the top view.In alternative embodiments, as shown in FIG. 3C (a top view), trenches26 may be long strips. In yet other embodiments, as shown in FIG. 3D(also a top view), trenches 26 form closed loops, which may encirclecenter 100_1 of chip 100, or encircle other selected points in chip 100other than center 100_1. The dimensions (such as lengths L1-L3 andwidths W1-W3 in FIG. 3B) and spacings S (such as spacings S1 and S2 inFIG. 3B) of different trenches 26 may be the same as each other, ordifferent from each other. Further, trenches 26 may be arranged in aperiodic pattern or a random pattern.

In the etch step for forming trenches 26, the trenches' profiles indifferent regions of chip 100 may be adjusted by various known etchingtechniques and effects. For example, the micro-loading effect is knownto affect the profiles for trenches having a width within a certainrange. Other known etch techniques can also affect trench profiles, forexample, using plasma bombardment or protective additives. Referringback to FIG. 3A, it is observed that slant angle α1 may be differentfrom slant angle α2, and slant angle α1 may be greater (or smaller) thanslant angle α2 by about 2 degrees, about 5 degrees, or even by about 10degrees. In addition, slant angle α2 may be greater (or smaller) thanslant angle α3 by about 2 degrees, about 5 degrees, or even greater thanabout 10 degrees. This may be achieved, for example, by adjusting widthW1, width W2, and width W3 relative to each other, depending on the etchtechnique used. The desirable pattern-loading effects may also beachieved by adjusting spacing S1 relative to spacing S2 and spacing S3.Accordingly, on chip 100, there exists a plurality of slant angles αthat is different from each other.

FIG. 4 illustrates a particular embodiment for forming trenches 26 (notshown in FIG. 4, please refer to FIGS. 3A-3D) with desirable slantangles. First, mask 28, which may comprise a polymer, is formed oversubstrate 20. Trenches 30 are formed in mask 28, for example, usinglaser micro-machining. The pattern of trenches 30 may be similar to thepattern of trenches 26. However, the aspect ratios of trenches 30 may bethe same as, or different from, the respective aspect ratios of trenches26. Mask 28 and the underlying substrate 20 are then etched using anetchant that attacks both substrate 20 and mask 28, with a desirableetching selectivity, for example, between about 0.5 and about 2. Duringthe etching process, the exposed portions of substrate 20 and mask 28are both etched when they are exposed to the etchant. However, inlocations where the mask 28 is thicker, it takes more time to etch mask28 before substrate 20 is exposed to the etchant. Accordingly, thepattern in mask 28 is transferred to substrate 20. In the embodimentswherein substrate 20 and mask 28 have a same etching rate (with etchingselectivity close to 1), the patterns in mask 28 are transferred to theunderlying substrate 20 substantially accurately. In the embodimentswherein substrate 20 has a greater etching rate than mask 28, trenches26 (not shown in FIG. 4, please refer to FIG. 3A) have higher aspectratios than the respective overlying trenches 30. Conversely, in theembodiments wherein substrate 20 has a lower etching rate than mask 28,trenches 26 have lower aspect ratios than the respective overlyingtrenches 30. The resulting textured substrate 20 is similar to thetextured substrate 20 shown in FIG. 3A. In yet other embodiments,instead of using etching to form textured substrate 20, lasermicro-machining may also be performed directly on substrate 20 to formtrenches 26 with desirable profiles.

Next, as shown in FIG. 5, LED 40 is formed on the textured top surfaceof textured substrate 20. In an exemplary embodiment, buffer layer 44 isformed on substrate 20 and may contact the textured top surface ofsubstrate 20. Buffer layer 44 may comprise un-doped gallium nitride. Aplurality of layers of LED 40 is then formed on buffer layer 44. LED 40may include cladding layer 46 of a first conductivity type over bufferlayer 44, at least one multiple quantum well (MQW) 48 that acts as anactive layer for emitting light over cladding layer 46, and claddinglayer 50 of a second conductivity type opposite the first conductivitytype over MQW 48. In an exemplary embodiment, cladding layer 46 is ann-GaN layer (GaN doped with an n-type impurity), MQW 48 is formed ofInGaN, and cladding layer 50 is a p-GaN layer (GaN doped with a p-typeimpurity). Top electrodes (which are also bond pads formed of metals) 52and 54 are formed to electrically connect to cladding layers 46 and 50,respectively. Accordingly, by applying a voltage between electrodes 52and 54, LED 40 may be activated to emit light. It is realized that LED40 may have many designs, which are also in the scope of the presentdisclosure. When the depths of trenches 26 are great enough, each oflayers 44, 46, 48, 50, 52, and 54 may have portions extending intotrenches 26. Alternatively, if trenches 26 are relatively shallow, onlythe lower ones of layers 44, 46, 48, 50, 52, and 54 have portionsextending into trenches 26, while upper ones of these layer are over topsurface 20 a of substrate 20, which means that each of layers 44, 46,48, 50, 52, and 54 comprises portions parallel to sidewalls 26_1,portions parallel to bottoms 26_2, and portions parallel to top surface20 a.

The formation methods of layers 46, 48, and 50 include epitaxial growth.Top electrodes 52 and 54 may be formed using a physical vapor depositionmethod, for example, sputtering, although other methods may also beused. The process details may be realized by one skilled in the art, andhence are not discussed in detail herein.

Layers 44, 46, 48, 50, 52, and 54 are non-flat layers, and may beconformal textured layers, which means that portions of these layers onsidewalls 26_1 of trenches 26 have substantially the same thickness asthe portions of the respective layers at bottoms 26_2 of trenches 26,and substantially the same thickness as the portions of the respectivelayers on the un-etched portions of top surface 20 a of substrate 20.Accordingly, each of layers 44, 46, 48, 50, 52, and 54 may follow theprofile of the textured top surface of substrate 20.

In the cross-sectional view as shown in FIG. 5, it is observed that dueto the different slant angles α (denoted as α1, α2, and α3) of sidewalls26_1, active layer 48 includes portions having different slant angles.Therefore, the light emitted from different portions of active layer 48with different slant angles α is directed to different directions. Forexample, FIG. 5 schematically illustrates five possible directions oflight (although light is also emitted to other directions), whichdirections are perpendicular to the top surfaces of the respectiveportions of active layer 48. With more trenches 26 having differentslant angles, the number of light directions is further increased.Accordingly, the light emitted by LED 40 is scattered more uniformly.

It is observed that with active layer 48 having a non-flat profile, thearea of active layer 48 is increased over the case if active layer 48 isa flat layer parallel to the top surface 20 a of substrate 20.Accordingly, the light output area is increased and the light amountemitted from a single chip is also increased. Further, the light emittedfrom the embodiments is scattered more uniformly, making it moresuitable for illumination applications, such as light bulbs. Thescattering of the light may be implemented by forming trenches directlyin the substrates on which LEDs are formed, rather than packagesubstrates. Accordingly, the embodiments provide a manufacturing processinvolving a low cost and low complexity.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method of fabricating a lighting apparatus,comprising: forming a mask layer over a substrate, the mask layer havinga plurality of protrusions that define a plurality of openings betweenthe plurality of protrusions; etching the substrate to form a pluralityof trenches in the substrate, wherein the trenches are aligned with theopenings of the mask layer, and wherein at least a first subset of thetrenches are shaped differently than a second subset of the trenches;and forming a multi-layered light-emitting diode (LED) structure overthe substrate; wherein: the multi-layered LED structure comprises afirst doped layer, a second doped layer doped with a different type ofconductivity from the first doped layer, and an active layer disposedbetween the first and second doped layers; and at least parts of the LEDstructure are formed over the trenches in the substrate.
 2. The methodof claim 1, wherein the openings extend completely through mask layerand expose portions of the substrate underneath before the substrate isetched.
 3. The method of claim 1, wherein the openings extend partiallythrough the mask layer and cover up portions of the substrate underneathbefore the substrate is etched.
 4. The method of claim 1, wherein atleast some of the openings have different lateral dimensions than otheropenings.
 5. The method of claim 1, wherein at least some of theprotrusions of the mask layer have different lateral dimensions thanother protrusions of the mask layer.
 6. The method of claim 1, whereinthe first subset of the trenches in the substrate have different slantangles than the second subset of the trenches in the substrate.
 7. Themethod of claim 1, wherein at least some of the first subset of thetrenches or the second subset of the trenches are shaped as rectanglesin a top view.
 8. The method of claim 1, wherein at least some of thefirst subset of the trenches or the second subset of the trenches areshaped as elongate strips in a top view.
 9. The method of claim 1,wherein at least some of the first subset of the trenches are encircledby at least some of the second subset of the trenches in a top view. 10.The method of claim 1, wherein the multi-layer LED structure is formedconformally over the substrate.
 11. A method of fabricating a photonicdevice, comprising: providing a substrate; forming, through alithography process, a plurality of recesses that extend into thesubstrate, wherein the recesses include a plurality of first recessesand a plurality of second recesses, and wherein at least some of thefirst recesses have different geometries than the second recesses;growing a first cladding layer over the substrate, the first claddinglayer having a first type of conductivity and partially filling therecesses in a conformal manner; growing a multiple quantum well (MQW)layer over the first cladding layer, the MQW layer being conformallygrown over the first cladding layer; and growing a second cladding layerover the MQW layer, the second cladding layer having a second type ofconductivity different from the first type and being conformally grownover the MQW layer.
 12. The method of claim 11, wherein the recesses areformed using an etching process.
 13. The method of claim 11, wherein therecesses are formed using a laser micro-machining process.
 14. Themethod of claim 11, wherein the geometries of the recesses areconfigured so that portions of the MQW layer overlying the recesses emitlight in different directions.
 15. The method of claim 11, wherein atleast one of the following is true: at least some of the first recessesor some of the second recesses are shaped as rectangles in a top view;at least some of the first recesses or some of the second recesses areshaped as elongate strips in a top view; and at least some of the firstrecesses are circumferentially encircled by at least some of the secondrecesses in a top view.
 16. A method of forming a device, the methodcomprising: providing a substrate; forming a first trench extending froma top surface of the substrate into the substrate, wherein the firsttrench comprises a first sidewall and a bottom, wherein the step offorming the first trench comprises a laser micro-machining; and forminga light-emitting device (LED) comprising an active layer, wherein theactive layer comprises a first portion parallel to the first sidewall ofthe first trench, and a second portion parallel to the bottom of thefirst trench.
 17. The method of claim 16, wherein the light-emittingdevice (LED) comprises: a first cladding layer of a first conductivitytype over the substrate; the active layer over the first cladding layer;and a second cladding layer of a second conductivity type opposite thefirst conductivity type over the active layer, wherein each of the firstcladding layer, the active layer, and the second cladding layer is anon-flat layer.
 18. The method of claim 16, wherein the step of formingthe first trench comprises etching the substrate.
 19. The method ofclaim 16, further comprising forming a second trench extending from thetop surface of the substrate into the substrate, wherein the secondtrench comprises a second sidewall having a second slant angle differentfrom a first slant angle of the first sidewall.
 20. The method of claim1, wherein the etching comprises a laser micro-machining process.